This invention relates to the measurement of blood flow in a subject, more particularly to a method and apparatus for measuring pulmonary blood flow by pulmonary exchange of oxygen and an inert gas with the blood utilising a divided respiratory system. The invention is especially suitable for monitoring pulmonary blood flow/cardiac output of a patient under general anaesthetic and accordingly it will be convenient to describe the invention in connection with this application. However, it is to be understood that the method and apparatus described herein may be used for determining the pulmonary blood flow or cardiac output of a subject in a conscious state.
The equation that links the cardiac output of a subject to more directly measured parameters is as follows:
{dot over (U)}gas={dot over (Q)}cxcex(FAgasxe2x88x92FVgas)
where FAgas refers to the concentration of inert soluble gas in the alveolar gas mixture of the lungs expressed as a fraction of its partial pressure to the barometric pressure (Bp),
F{overscore (V)}gas refers to the fraction of the inert soluble gas in the mixed venous blood expressed as a fraction of its partial pressure to the total pressure,
xcex is the Ostwald solubility coefficient of the inert soluble gas in blood,
{dot over (Q)}c is the cardiac output that passes through the pulmonary capillaries in the walls of gas-containing alveoli, and
{dot over (U)}gas is the uptake into the blood from the alveoli measured in units of volume at body temperature and barometric pressure per unit time.
This equation holds true for inert gases only. In this regard an inert gas dissolves in blood proportionally to its partial pressure i.e. it obeys Henry""s Law. By contrast a reactive gas does not obey Henry""s Law by reason of its reacting chemically with blood constituents. Oxygen and carbon dioxide are examples of reactive gases.
The term cardiac output as used herein refers to the amount of blood per unit time which passes through the pulmonary capillaries in the walls of the alveoli of the lungs. If haemoglobin O2 saturation of the subject is 100% then the whole cardiac output will be equivalent to the pulmonary blood flow, i.e. the amount of oxygenated blood passing through the pulmonary capillaries in the walls of the alveoli of the lungs. If this saturation is less than 100% the whole cardiac output includes shunt blood in addition to pulmonary blood flow. Shunt blood does not transport O2 from the lungs to the tissue and may therefore be ignored. The % shunt may be estimated from pulse oximetry.
Most methods in use today or described in the literature refer to or depend on the above equation, but F{overscore (V)}gas cannot be measured accurately without obtaining a sample of mixed venous blood, which would sacrifice the advantage of non-invasiveness of large blood vessels with catheters, as is necessary with the most widely used method of measuring cardiac output presently in use, namely the thermodilution method.
Most gas exchange methods for measuring the cardiac output which have been attempted suffer from the problem of xe2x80x9crecirculationxe2x80x9d which limits them to only intermittent determinations of {dot over (Q)}c separated by relatively long intervals to wash out gas introduced by the previous determination. This restriction of frequency of taking readings of {dot over (Q)}c is necessary to ensure that P{overscore (V)}gas has returned to a value close to zero before another determination is performed. The same constraint also applies to methods using reactive gases. The term xe2x80x9crecirculationxe2x80x9d refers to the return back to the lungs in the mixed venous blood of gas that has previously been taken away from the lungs in the arterial blood.
It is an object of the present invention to overcome or at least alleviate one or more of the abovementioned difficulties of the prior art, or at least to provide the public with a useful choice.
Accordingly in a first aspect the present invention provides a method for measuring the pulmonary bloodflow in a subject including:
isolating two or more divisions of the respiratory system, said divisions comprising the complete gas exchanging part of said respiratory system,
ventilating each said division with a separate gas mixture, at least one of said gas mixtures including an inert soluble gas,
determining uptake of inert soluble gas in at least two of said divisions,
determining uptake of oxygen in each of said divisions,
determining end tidal concentration of inert soluble gas in at least two of said divisions, and
calculating pulmonary bloodflow from determined values of uptake and end tidal concentration of inert soluble gas, and uptake of oxygen.
Preferably two or three divisions of the respiratory system are isolated, most preferably three divisions.
When three divisions are isolated it is preferred that two of the divisions are ventilated with gas mixtures which are substantially balanced with respect to inert soluble gas, the concentration of inert soluble gas in each of these two divisions being different from each other, and the third division is ventilated with a gas mixture which is unbalanced with respect to inert soluble gas.
One method of isolating two or more divisions of the respiratory system utilises a multi-lumen cuffed endobronchial catheter.
Accordingly in a second aspect of the invention there is provided apparatus for measuring pulmonary bloodflow in a subject including:
a multi-lumen cuffed endobronchial catheter adapted to allow separate gas mixtures to be provided to two or more separate divisions of the respiratory system of the subject, said separate divisions comprising the complete gas exchanging part of said respiratory system,
two or more breathing systems for supplying different mixtures to each lumen of said multi-lumen catheter at the same rate and the same total pressure,
two or more gas delivery systems for providing gas mixtures to said two or more breathing systems,
sampling means for sampling (i) inspired and expired gas in each division and/or (ii) the fresh flow gas and the exhaust gas of each division, and
gas analyser for determining concentrations of gases in said samples,
a flow determining means for determining flow rates of (i) said inspired and expired gas and/or (ii) said fresh flow gas and exhaust gas and,
processing system for calculating pulmonary blood flow from said determined concentrations and flow rates.
One method of ensuring that the gas mixtures are supplied to each lumen at the same rate and the same total pressure is to use a bag-in-a-box type ventilator with each breathing system, and operate the ventilator by supplying a common working gas. Other methods for synchronising the rate and pressure of mixed gas supplied to the lumens of the catheter would be evident to a person skilled in the art.
It is to be noted that the defined apparatus is not essential for use of the method of measurement defined above, but represents a particularly convenient apparatus useful in making the required measurements.
Endobronchial catheters having more than two lumens are novel and represent a third aspect of the present invention. Particularly accurate results can be obtained if the multi-lumen cuffed endobronchial catheter has three lumens.
Accordingly in a fourth aspect of the invention there is provided a triple lumen, cuffed endobronchial catheter for providing separate gas mixtures to each of three separate divisions of the respiratory system of a subject, said three divisions comprising the complete gas exchanging part of said respiratory system, said catheter comprising:
a primary tube having three lumens adapted to be inserted within the trachea of a subject, each of said lumens opening at a top end thereof into a connector tube adapted to be connected to a breathing system, and opening at a bottom end thereof into an outlet for delivering a gas mixture to one of said divisions,
one or more inflatable cuffs located about said primary tube and/or said outlets adapted to form seals within the respiratory system such that each outlet is capable of delivering a gas mixture to one of said three separate divisions in isolation from each of the other divisions.
The outlet may be an opening in a tube, or a short tube with an opening, for delivering a gas mixture to a division of the respiratory system from a lumen of the primary tube. The outlet may be an extension of a lumen of the primary tube or may be an opening in the bottom end of a lumen.
The triple lumen cuffed endobronchial catheter preferably has an inflatable cuff located about the primary tube and above the outlets which is adapted to form a seal within the trachea.
In a particularly preferred embodiment the triple lumen catheter includes a first inflatable cuff as described above in combination with a second inflatable cuff located between the first and third outlets for forming a second seal in the right bronchus and a third seal in the hyparterial bronchus, the third seal allowing the third outlet to provide a gas mixture to the middle and lower lobes of the right lung and the second and third seals together allowing the second outlet to provide a gas mixture to the upper lobe of the right lung.
The second inflatable cuff preferably encircles the second outlet and lies within the right main bronchus and the hyparterial bronchus.
It is also possible to manufacture triple lumen catheters with inflatable cuffs as described above which are adapted to supply gas mixtures to the right lung, the upper lobe of the left lung and the lower lobe of the left lung, although for technical reasons it is less convenient.